In recent years, high-efficiency transmission using digitization has been employed in radio communications. Usually, a radio apparatus that performs such radio communications includes a power amplifier. The radio apparatus inputs a transmission signal to the power amplifier and then emits the transmission signal, whose power is amplified and which is output from the power amplifier, to the atmosphere via an antenna. Hereinafter, the power of the signal that is input to the power amplifier can be referred to as an “input power” and the power of the signal that is output from the power amplifier can be referred to as an “output power”.
Such a power amplifier has a characteristic that, when the input power is larger than a certain value, the relation between the input power and the output power is not liner. This characteristic will be described here using FIG. 9. FIG. 9 is a graph of an example of I/O (input/output) characteristics of the power amplifier. The horizontal axis in FIG. 9 represents the power of the signal that is input to the power amplifier and the vertical axis in FIG. 9 represents the power of the signal that is output from the power amplifier.
In the example illustrated in FIG. 9, when the input power is smaller than a certain value “PX”, the relation between the input power and the output power is linear. In contrast, when the input power is larger than the certain value “PX”, the relation between the input power and the output power is not linear. Specifically, when the input power is larger than the certain value “PX”, the output power is saturated. As described above, the I/O characteristics of the power amplifier can be divided into a “liner area” in which the relation between the input power and the output power is linear and a “non-linear area” in which the relation between the input power and the output power is not linear.
The signal that is output from the power amplifier having the above-described non-linear area contains a non-linear distortion, which leads to a problem that the communication quality deteriorates. This problem will be described here using FIG. 10. FIG. 10 is a graph of an example of frequency spectrums. The horizontal axis in FIG. 10 represents the frequency and the vertical axis in FIG. 10 represents the power. The solid line L11 in FIG. 10 represents the frequency spectrum of the signal on which power amplification is performed in the non-linear area and the dotted line L12 in FIG. 10 represents the frequency spectrum of the signal on which power amplification is performed in the linear area.
As illustrated in FIG. 10, sidelobe increases in the power of the signal on which power amplification is performed in the non-linear area compared with the power of the signal on which power amplification is performed in the liner area, and thus a power leakage to adjacent channels occurs. This is because a signal on which power amplification is performed in a non-linear area contains more non-linear distortions compared with a signal on which power amplification is performed on a liner area. Such power leakage deteriorates the communication quality of adjacent channels.
Some recent radio apparatuses include a distortion corrector that corrects a non-linear distortion contained in a transmission signal in order to prevent deterioration of the communication quality. Specifically, the distortion corrector performs a distortion correction process on an input signal, which is input to a power amplifier, using a distortion correction coefficient that is stored in a predetermined storage unit. The distortion corrector calculates an update value of the distortion correction coefficient according to the input signal, which is input to the power amplifier, and a feedback signal that is fed back from the power amplifier. The distortion corrector then updates the distortion correction coefficient, which is stored in the predetermined storage unit, to an update value of the distortion correction coefficient.    Patent Document 1: Japanese Laid-open Patent Publication No. 2008-219674    Patent Document 2: International Publication Pamphlet No. WO 2003/103163
However, using a conventional technology in which the above-described distortion correction process is performed leads to a problem that the power range of the transmission signal ensuring the communication quality narrows. This problem will be described here using FIG. 11.
FIG. 11 is a graph of the relation between the gain of the power amplifier and the expected value of the distortion correction coefficient. The horizontal axis in FIG. 11 represents the power of the signal that is input to the power amplifier and the vertical axis in FIG. 11 represents the gain of the power amplifier or the expected value of the distortion correction coefficient. The phrase “expected value of the distortion correction coefficient” means an optimum value of the distortion correction coefficient that is used for the distortion correction process or the average value of optimum values of the distortion correction coefficient. The solid line L13 in FIG. 11 represents the gain of the power amplifier and the dotted line L14 in FIG. 11 represents the expected value of the distortion correction coefficient. The numerical values above the dotted line L14 denote an example of distortion correction coefficients.
As illustrated in the example in FIG. 11, if the input power is larger than a certain value, the output power is saturated and thus the gain of the power amplifier decreases. Thus, as represented by the dotted line L14 in FIG. 11, it is desirable that, when the input power is larger than the certain value, the distortion corrector perform the distortion correction process using a larger distortion correction coefficient as the input power increases. However, because the distortion corrector stores distortion correction coefficients respectively for the ranges of the input power, the distortion correction process may possibly not be performed using an optimum distortion correction coefficient on each input power not as in the case represented by the dotted line L14 in FIG. 11. This aspect will be described below.
In the example illustrated in FIG. 11, the distortion corrector stores “1.0” as the distortion correction coefficient corresponding to the input power “P10” to “P20”. Similarly, the distortion corrector stores “1.0” as the distortion correction coefficients corresponding to the input powers “P20” to “P30”, “P30” to “P40”, “P40” to “P50”, “P50” to “P60”, and “P60” to “P70”. The distortion corrector also stores any value of “1.0” to “1.5” as the distortion correction coefficient corresponding to the input power “P70” to “P80”. The distortion corrector also stores any value of “1.5” to “4.0” as the distortion correction coefficient corresponding to the input power “P80” to “P90”. Hereinafter, “Pxx” to “Pyy” means equal to or larger than “Pxx” and smaller than “Pyy”.
It is assumed that the expected value of the distortion correction coefficient with respect to the signal having the input power “P81” is “1.7” and the expected value of the distortion correction coefficient with respect to the signal having the input power “P82” is “3.8”. In addition, it is assumed that the signal having the input power “P81” is input to the power amplifier and accordingly the distortion corrector gradually updates the distortion correction coefficient corresponding to the input power “P80” to “P90” to “1.7”. When a signal having the input power “P82” is then input to the power amplifier, the distortion corrector performs the distortion correction process using the distortion correction coefficient “1.7” until the distortion correction coefficient is updated. However, it is desirable that the distortion correction process be performed on the signal having the input power “P82” using the distortion correction coefficient “3.8”. For this reason, the above-described distortion correction process may not sufficiently correct the non-liner distortion contained in the signal.
It is assumed that, in the above example, the signal having the input power “P82” is input to the power amplifier and accordingly the distortion corrector gradually updates the distortion correction coefficient corresponding to the input power “P80” to “P90” to “3.8”. Thereafter, when a signal having the input power “P81” is input, the distortion corrector performs the distortion correction process using the distortion correction coefficient “3.8”. However, it is desirable that the distortion correction process be performed on the signal having the input power “P81” using the distortion correction coefficient “1.7”. For this reason, the above-described distortion correction process may not sufficiently correct the non-liner distortion contained in the transmission signal.
The same may possibly occur regarding the input power “P70” to “P80”. For example, the distortion corrector may possibly perform a distortion correction using a distortion correction coefficient “1.1” on a signal regarding which it is preferable that a distortion correction is performed using a distortion correction coefficient “1.4”. However, because the difference between the distortion correction coefficient “1.4” and the distortion correction coefficient “1.1” is small, the distortion corrector may possibly correct the non-liner distortion contained in the signal. In other words, the distortion corrector may possibly not sufficiently correct the non-linear distortion contained in the transmission signal if the range of the distortion correction coefficient is broad as in the case of the input power “P80” to “P90”.
For this reason, in the example illustrated in FIG. 11, when the power of the signal that is input to the power amplifier is equal to or larger than “P80”, the non-linear distortion may possibly not corrected in the signal that is output from the power amplifier. If the power of the signal that is input to the power amplifier is equal to or larger than “P80”, the radio apparatus including the power amplifier that has the characteristics illustrated in FIG. 11 may not ensure the communication quality. In other words, in order to transmit a signal ensuring the communication quality, the radio apparatus limits the power of the signal that is input to the power amplifier to be smaller than “P80”. For this reason, in the conventional technology in which the distortion correction process is performed, the power range of the transmission signal to ensure the communication quality narrows and thus the performance of the power amplifier may not be sufficiently brought out.
A technology has been recently proposed in which no distortion correction coefficient is generated and updated when the instantaneous power of a transmission signal exceeds a threshold. However, it is still difficult to solve the above-described problem using this technology. Specifically, this technology does not update the distortion correction coefficient, therefore, there is a risk that a non-linear distortion contained in the transmission signal is not corrected in the distortion correction process.